This invention relates to a thermal mass controller or flow meter, and more particularly to an improved concentric rod and tube sensor for such a thermal mass flow meter.
In general, thermal mass flow meters are instruments for accurately measuring fluid flow rates, preferably gas flow rates. Typically, such instruments include a flow sensor which generates an output signal proportional to the flow rate of the fluid flowing through the thermal mass flow meter and this output signal can be used for indicating, recording, or controlling the flow rate of the fluid. Oftentimes, the thermal mass flow meter is combined with an automatic controller which regulates the flow to a predetermined flow rate level and thus eliminates the need for continuous monitoring and manual readjustment of fluid pressure so as to provide a uniform gas or fluid flow rate. In one type of known thermal mass flow meter, the sensor is a hollow tube having a heater coil wound therearound. On the inlet and outlet sides of the heater coil, sensor coils are provided on the tube. The heater coil generates a uniform amount of heat which is conducted through the sensing tube to the fluid flowing through the tube. The sensor coils are connected to a balancing bridge and thus detect the resulting temperature differential of the fluid flowing through the sensing tube with the temperature differential being proportional to mass flow rate. Such thermal mass flow meters are commercially available from the Brooks Instrument Division, Emerson Electric Co. of Hatfield, Pa. under the model number DS-5810.
It will be appreciated that the performance of such thermal mass flow meters is linked closely to the heat transfer between the heated portion of the sensing tube and the fluid flowing therethrough. Further, the heat transfer coefficient of the fluid flowing through the sensor tube is highly dependent on boundary layer conditions between the fluid and the surfaces of the sensor tube in contact with the fluid being measured. Oftentimes thermal mass flow controllers are utilized to provide a steady flow of gas to a process, such as is used in the processing of semiconductor materials. Also, such mass flow controllers are utilized to control carrier gasses in gas chromatographs. Typically, the flow rates of these gasses are relatively low, for example, 0-500 sccm.
In such applications, the flow rate of gasses, such as silane, must be accurately regulated. These gasses have a tendency to plug or coat small diameter flow passages within the thermal mass flow meter. However, so as to establish proper boundary layer conditions for accurately regulating the mass flow rate of the gasses, it is necessary to maintain close and precise tolerances within the sensing tube. In an effort to overcome the tendency of small diameter orifices in prior mass flow meter sensor tubes, a so-called concentric rod and tube sensor was developed and has been commercially available from the Brooks Instrument Division, Emerson Electric Co., Hatfield, Pa. as model number DS-5858.
This concentric rod and tube sensor utilized a relatively thin wall outer tube which was sealably fitted within the flow passage through the flow body of the flow meter. The heater coil and sensor coils were applied to the outer surface of the tube in heat transfer relation therewith. At spaced locations along the sensor tube, groups of three inwardly projecting dimples or depressions were formed spaced around the circumference in the tube at the same diametric plane. These dimples or depressions engaged the outer surface of a rod inserted within the tube so as to concentrically locate the rod within the tube and so as to establish a concentric annular flow passage between the inner surface of the tube and the outer surface of the rod. This concentric tube and rod sensing element provided a straight through measuring tube which effectively controlled boundary layer conditions of the gas flowing through the tube and yet permitted the tube and the rod to be readily removed from the flow meter body for periodic cleaning. This concentric annulus overcame the problem of prior thermal mass flow meter using small diameter orifices of becoming plugged.
However, it was difficult to accurately form the depressions in the tube of this prior flow meter so as to precisely establish the concentric relationship of the rod within the tube. Also, it was found during shipping of the flow meter, the rod was oftentimes not positively held in concentric relation within the tube such that the rod would be displaced toward one side of the tube resulting in the annular flow passage becoming eccentric rather than concentric. This eccentricity of the flow passage could so adversely affect the calibration of the instrument such that it would not accurately regulate or control the flow rate of gasses as intended. Also, during disassembly and reassembly of the instrument for cleaning purposes, it was sometimes found that calibration would be affected due to a resulting eccentricity of the flow passage because the rod could not be precisely maintained in coaxial relation with the sensor tube upon re-assembly of the rod within the tube. Still further, it was found that in use, the dimensions of the dimples or depressions in the sensor tube would vary. This was caused by repeated removal and insertion of the rod for cleaning purposes placing a strain on the depressions or dimples which resulted in a permanent deformation thereof. Thus, after time, it was found that an eccentric flow passage would result with a corresponding shift in the calibration of the flow meter.